1. Field of the Invention
This invention relates to a method of and an apparatus for ashing away a photo-resist of a work. The invention is applicable to, for instance, photo-resist separation in a semiconductor integrated circuit manufacture process and, particularly, is suitably applicable for satisfactorily separating a photo-resist after high dose ion implantation without leaving a residue or the like.
2. Description of the Relevant Art
For the photo-resist separation in semiconductor processes, a commonly termed dry ashing process, which utilizes O.sub.2 plasma, has become extensively adopted in mass production sites as well as in place of a wet process using fuming nitric acid or sulfuric acid hydrogen peroxide (i.e., a blend solution of sulfuric acid and hydrogen peroxide).
The basic mechanism of the dry ashing process is to remove photo-resist, which comprises an organic polymer, by ashing the photo-resist into CO and CO.sub.2 through a combustion reaction caused by providing O radicals, O.sub.2 radicals, and so forth, generated in plasma. The process permits comparatively ready separation of the usual photo-resist materials.
However, there are cases where it is difficult to remove photo-resist material. For example, photo-resist after ion implantation can not always be readily separated. Particularly, the problem is significant when a high dose ion implantation is necessary as in the formation of source/drain regions for super-LSI manufacture.
Where a photo-resist is used as an ion implantation mask, high dose, high energy ion implantation is made into the photo-resist as well. At this time, the photo-resist surface is hardened mainly presumably due to heat generated by the ion bombardment. The hardened layer can not be readily removed with, for instance, the usual O.sub.2 plasma alone, thus extremely deteriorating the separability of the photo-resist.
It is thought that the hardened surface layer is formed not only by heat, but also in that the ion implantation dopant effects substitution in the molecular structure of the photo-resist material to cause a cross-linking reaction, the reacted portion being oxidized by O.sub.2 plasma to remain as residue in a layer which is difficult to etch. Particularly, such a residue is liable to constitute a source of particle contamination to extremely reduce the yield of super-LSI or like product.
To solve this problem, a commonly termed two-step ashing process has been proposed, which comprises two steps of removing the above hardened layer in a RIE process incorporating an H.sub.2 type and using ion bombardment and then carrying out ordinary ashing (Fujimura et al, "Applied Physics", spring 1989, 1P-13, p. 574). This process, however, involves an additional step and dictates an elaborate system. Further, the H.sub.2 RIE process requires a long time and therefore reduces the throughput. Accordingly, further improvement of the process has been demanded.